1. Field of the Invention
This invention relates to a mirror supporting device for supporting a mirror in a suitable attitude while maintaining a mirror surface in a predetermined shape by compensating for an influence of gravity.
2. Description of the Prior Art
FIG. 1 is a structural view showing a mirror supporting system using a conventional mirror supporting device; FIG. 2 is a plan view showing a mirror supported at a suitable angle .theta. of inclination by said mirror supporting device; and FIG. 3 is a structural view showing a conventional mirror supporting device.
In FIG. 1, reference numeral 1 designates a spherical mirror made of glass having a concave mirror surface, the mirror 1 having a hole 1a in a central portion thereof and being secured at three fixing points a, b and c as also shown in FIG. 2. Reference numeral 2 designates a mirror cell for supporting the mirror 1; 3 a plurality of mirror supporting devices for supporting the mirror 1 in a suitable attitude on the mirror cell 2; 4 a mirror fixing mechanism for fixing the mirror 1 at said fixing points a, b and c on the mirror cell 2; 5 a plurality of force control devices for controlling a support force of each of the mirror supporting devices 3 in accordance with a force command value 6a and a detection signal 11a from an axial support force detector (described later) provided on each of the mirror supporting devices 3; and 6 a force command value calculating device for calculating said force command value 6a applied to each control device 5 in response to said angle of inclination .theta..
In FIG. 3 showing the aforesaid mirror supporting device 3, reference numeral 1b designates a recess portion provided in the back surface of the mirror 1; 2a a recess portion provided in the mirror cell 2; 7 a lever provided in the recess portion 2a and extending to the recess portion 1b; 8 a counter weight provided on one end of the lever 7; 12 a guide mechanism provided movably in an axial direction of the lever 7 within the recess portion 2a; 13 a universal joint for connecting an intermediate portion of the lever 7 to the guide mechanism 12; 14 a spring connected to the guide mechanism 12; 15 a ball-screw device with a motor controlled by the force control device 5 shown in FIG. 1, for enabling the guide mechanism 12 together with the lever 7 to move via the spring 14; 9 a guide mechanism provided movably in an axial direction of the lever 7 within the recess portion 1b; 10 a universal joint for connecting the other end of the lever 7 to the guide mechanism 9 at the center of gravity g of the mirror 1; and 11 an axial support force detector provided in the recess portion 1b to detect an axial support force.
The apparatus operates as follows.
First, in FIG. 2, the mirror 1 is fixed to the mirror cell 2 at three fixing points a, b and c by the mirror fixing mechanisms 4. In order to prevent own-weight deformation of mirror 1 caused by gravity, a plurality of mirror supporting devices 3 are provided to generate a force in a direction of counteracting the mirror's own weight. Each mirror supporting device 3 is controlled by a force control device 5 so as to provide a force corresponding to the imparted force command value 6a. What support force should be generated in each of the mirror supporting devices 3 is determined by the force command value calculating device 6 in response to the angle of inclination .theta. of the mirror 1.
Next, the principle of the mirror supporting device 3 shown in FIG. 3 will be described.
Since the mirror 1 is inclined to a suitable attitude, it is necessary to apply forces of two directions in order to counteract the weight of the mirror 1. One is a force at right angle to the surface of mirror 1 (axial force), and the other is a force in a planar direction of the mirror 1 (radial force). First, how the radial force is generated will be described.
Assuming W is a shared weight of the mirror 1 being charged upon each mirror supporting device 3, the lever ratio of the lever 7 (the ratio between the distance from the universal joint 10 to the universal joint 13 and the distance from the universal joint 13 to the counter weight 8) and the weight of the counter weight 8 are then determined so as to provide a force equal to W. Each universal joint 10 can be supported in the recess portion 1b provided in the mirror 1 so as to be positioned at the center of gravity g of the mirror 1 to thereby counteract the radial force acting on the mirror 1 due to gravity.
With respect to the counteract axial force, a force is generated by the spring 14 and the ball-screw device with motor 15, and the force is transmitted to the mirror 1 through the guide mechanism 12, the universal joint 13, the lever 7, the universal joint 10, the guide mechanism 9 and the axial support force detector 11. The detection signal from the axial support force detector 11 is fed back to the force control device 5, and the force control device 5 controls the ball-screw device with motor 15 so that the axial support force becomes equal to the gravitational axial force, as determined by force command value 6a.
Since the conventional mirror supporting device 3 is constructed as described above, the recess portions 1b have to be provided in the mirror 1 in order to support the mirror 1 at its center of gravity g. Thus, difficult processing is required in which the mirror 1 made of glass is bored with holes to form the recess portions 1b. In addition, there involved problems such that cracks remain even after processing, and the reliability in strength is materially lowered.